Project Summary/Abstract: The commensal Bacteroides spp. account for about 30-40% of total bacteria in the human colon. This environment is a highly competitive ecosystem and in order for Bacteroides spp. to maintain their high cell number, they need to compete efficiently for the available nutrients such as iron with other microorganisms of the microflora. Nutritional iron has a remarkable influence on the gut microflora. The competition for iron changes the commensal microflora and iron foraging by commensals protect against colonization by pathogens and colitis. This supports a role for Bacteroides spp. in intestinal iron homeostasis. However, the iron acquisition mechanisms utilized by Bacteroides spp. to compete for iron in the complex microbial ecosystem of the human colon have not been examined. In this application we show that Bacteroides fragilis utilize ferric iron-bound siderophores produced by other organisms (xenosiderophores) to grow under iron limiting conditions anaerobically. In addition, we have identified a TonB-dependent outer membrane transporter present only in B. fragilis, BfFchA1, involved in the utilization of ferric iron-bound siderophore for growth under anaerobic conditions. Moreover, we propose that iron is dissociated/reduced from the ferric iron-siderophore complex in the periplasmic space because the internalization of iron is dependent on the transmembrane ferrous iron transporter system FeoAB. Thus, these studies will expand our understanding of B. fragilis iron homeostasis and its contribution to intestinal colonization. For this application, our hypothesis is that BfFchA1 and FeoAB are critical for B. fragilis iron acquisition during gut colonization. To test our hypothesis, we will in aim 1) define the role of BfFchA1 in ferri-siderophore assimilation anaerobically and in aim 2) define the role of BfFchA1 and FeoAB in B. fragilis intestinal colonization in a mouse model. The long term goal of these studies is to provide targets that could lead to the development of new probiotic therapeutic strategies for the prevention or treatment of intestinal disorders in view of B. fragilis beneficial immunomodulatory properties.